In order to recover the transmitted digital signals from two conjugate carrier waves of like frequency identifying n different signal levels by their relative amplitudes, it is known to demodulate these carriers with the aid of coherent detectors having control inputs connected to sources of two oscillations of carrier frequency in relative phase quadrature, these oscillations thus being of the form Kcos.omega..sub.c t and Ksin.omega..sub.c t where K is a constant and .omega..sub.c is the pulsatance of the carrier waves. With, say, oscillation Ksin.omega..sub.c t suitably synchronized with a carrier oscillator at the transmitting end to define a reference axis, the coherently demodulated carriers give rise to two input voltages defining respective coordinates of an orthogonal matrix locating a point on that matrix which lies on a radius including with the reference axis a certain angle .theta. adapted to assume-- ideally-- any of n different values. In practice, this angle will vary on reception within certain tolerance limits about the n nominal values of .theta..
These variations are due, at least in part, to two kinds of distortions occurring in such two-channel systems, namely an intrachannel distortion between signals transmitted in successive cycles and an interchannel distortion resulting from the interaction of substantially concurrently transmitted signals on the two channels. For a discussion of this general problem, in a somewhat different system using quadrature amplitude modulation (QAM), reference may be made to an article by D. D. Falconer and G. J. Foschini entitled "Theory of Minimum Mean-Space-Error QAM Systems Employing Decision Feedback Equalization", Bell System Technical Journal, December 1973, page 1821.